Any web application can have a security bug, which exploited, may allow the attacker to gain unauthorized access to crucial application components, and, unfortunately, lead to a leakage of a sensitive data. One of the common ways to defend the zero days is to use WAF (Web Application Firewall). In this tutorial I'll put some light on a dedicated Nginx WAF external module called NAXSI (Nginx Anti XSS & SQL Injection), the ModSecurity equivalent for Apachers, by the way...